1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use in an image forming apparatus such as a copying machine or a printer, and more particularly to a device for heating an image with heat transmitted from heater across a film.
2. Related Background Art
In the electrophotographic image forming process, the fixation of the toner image developed on the recording material has generally been achieved by heat and pressure in the heat roller system while the recording material is pinched between and transported by a heating roller controlled at a predetermined temperature and a pressure roller having an elastic layer and maintained in pressurized contact with the heating roller.
Recently, for power saving in the stand-by state and for reducing the time from the start of power supply to the image output, there has been proposed, as disclosed in the Japanese Patent Laid-Open Application Nos. 63-313182 and 2-157878, the fixing device of a film heating fixation method comprising a heater unit including at least a fixed heating member (heater) and a heat-resistant film (fixing film) which is transported in contact with the heater, and a pressing member for maintaining the recording material in close contact with the surface of the heat-resistant film of the heater unit, wherein the toner image formed on the surface of the recording material is fixed by the heat supplied to the recording material from the heater though the film.
FIG. 5 schematically shows an image fixing device, based on the conventional film heating fixation method mentioned above. Referring to FIG. 5, a heater unit 60 is composed of a heat-resistant film 65 of a substantially cylindrical form, a heater 61 constituting the heating member, a film guide 66 provided inside the heat-resistant film 65 and constituting a heater stay for fixing the heater 61, a temperature detecting element 63 maintained in contact with the heater 61 and adapted to detect the temperature thereof, and an inverse U-shaped reinforcing metal plate 67. A pressure roller 77, composed of a metal core 71 and silicone rubber 73 and rendered rotatable, is maintained in pressure contact with the heater unit 60 to constitute the image fixing device. In maintaining the pressure roller 77 in contact with the heater unit 60 under pressure, the reinforcing metal plate 67 provided on the heater stay 66 of the heater unit 60 prevents the heater 61, thermistor 63, heater stay 66 etc. from deformation by the contact force of the pressure roller 77. The temperature detecting element 63 can be composed, for example, of a thermistor.
The heat-resistant film 65 of the substantially cylindrical form is composed of a substrate layer of a polyimide film of a thickness of 40 to 60 .mu.m, and a releasing layer of a thickness of 5 to 20 .mu.m, provided on the external peripheral surface (coming into contact with the recording material and the toner image) and consisting of PFA and a dispersion of PTFE in PFA. The heat-resistant film 65 is so constructed as to have an internal peripheral length larger than that of the film guide 66 and the reinforcing metal plate 67, whereby the film guide 66 and the reinforcing metal plate 67 can be positioned inside the heat-resistant film 65.
The heater 61 is composed of an insulating, heat-resistant ceramic substrate of a low heat capacity, elongated in a direction perpendicular to the transport direction of the recording material P, and a heat-generating resistance member 62 printed on the surface of the substrate along the longitudinal direction thereof. The temperature detecting element 63 is maintained in contact with a side of the ceramic substrate, opposite to the exposed surface of the heat-generating resistance member 62. The heater 61 is so fixed, under thermal insulation, to the film guide 66 formed with a semi-circular cross section, as to expose the surface of the heat-generating member. The temperature detecting element 63 maintained in contact with the heater 61 is connected to a CPU 101, which drives a triac 55 according to the temperature detection output of the temperature detecting element 63 to control the current supply from a power source 35 to the heat-generating member 62, thereby controlling the temperature of the heater 61.
The pressure roller 77 is pressed to the heater unit 60 with a total pressure of 9 to 11 kgf by pressurizing means (not shown) and is rotated counterclockwise, along the transporting direction of the recording material P, by drive means (not shown). By the rotation of the pressure roller 77, the heat-resistant film 65 of the heater unit 60 rotates around the film guide 66, while it slides on and in close contact with the surface of the heat-generating member of the heater 61. In order to reduce the sliding friction between the heater and the internal surface of the film, heat-resistant grease is provided therebetween.
In the image fixing device of the above-described configuration, the recording material P is subjected to the fixation, by pressure and fusion, of the toner image supported thereon, while it is guided between the heat-resistant film 65 and the pressure roller 77 by transport means and passed through a fixing nip therebetween while the heater 61 is heated to a predetermined temperature.
The above-described film heating fixation method allows to reduce the heat capacity of the heater to a few per cent of that in the fixing device of the heat roller method, and also to employ a heat-generating member of fast temperature rise, thereby enabling the heater to reach the fixation temperature within a short time in the order of several seconds. It is therefore possible not to effect power supply to the fixing device during the stand-by state but to start the power supply after the recording material is picked up in the course of the image forming operation, thereby saving the power consumption and shortening the start-up time of the equipment.
With such film heating fixation device, however, in case a printing operation is started in a state in which the pressure roller etc. are at the room temperature (printing operation is hereinafter called cold start if the power supply to the heat-generating member is started from a state where the pressure roller is at the room temperature), if a sheet of paper left under a high humidity environment is supplied, the vapor generated from the paper under heating condenses on the surface of the pressure roller, thereby significantly reducing the transporting power thereof and causing slippage between the paper and the pressure roller.
Also in the cold start operation mentioned above, the temperature of the pressure roller is higher in the film fixation method than in the heat roller fixation method, so that the temperature is controlled somewhat higher, than in the heat roller fixation method, for the initial several sheets (until the temperature of the pressure roller is sufficiently elevated) in order to fix the image solely with the heat from the heater unit. For this reason, in the film heating fixation device of a configuration in which the distribution of heat generation of the heater surface substantially coincides with the center of pressurization, the viscosity of the toner becomes lower particularly at the downstream side of the fixing nip, thereby eventually resulting in so-called hot offset phenomenon in which the toner is peeled off from the surface of the recording material. Such phenomenon tends to occur more often in thin paper or in high-grade paper.
Also in case a line image is formed on the moist recording material, such image may scatter by the vapor generated at the image fixation and deposit on the recording material, thus smearing the obtained copy. Such scattering phenomenon usually occurs in the downstream side of the image and is therefore called trailing edge.